1. Field of the Invention
The invention is directed to a heat stabilized thermoplastic polyester molding resin. The instant invention is directed to a long term heat stabilized polyalkylene terephthalate resin composition. More specifically, the instant invention is directed to a long term heat stabilized polyalkylene terephthalate composition consisting of a polyalkylene terephthalate resin, a reinforcing filler and stabilizing amounts of the combination of polyamide polymer, phenoxy resin and an organic phosphonate.
2. Description of the Prior Art
The surprisingly superior results attainable by employing a reinforced polyalkylene terephthalate resin selected from the group consisting of polypropylene terephthalate and polybutylene terephthalate as a molding resin has recently been recognized in the art such as U.S. Pat. No. 3,814,725. These superior results include improved moldability as well as improved properties of the reinforced polyalkylene terephthalate molded article.
As is well known in the art, reinforced polypropylene terephthalate and polybutylene terephthalate can be molded and otherwise processed at lower temperatures and have a significantly shorter cycle time in the mold than reinforced polyethylene terephthalate. More importantly, these resins do not require, as does reinforced polyethylene terephthalate, the presence of a nucleating agent to induce crystallinity. It has recently been discovered that nucleating agents and/or internal mold release agents when added to reinforced polypropylene terephthalate or reinforced polybutylene terephthalate molding resins cause a marked decrease in surface quality i.e., the glossy surface finished on products molded therefrom is rendered rough, streaky and uneven. Thus, polypropylene terephthalate and polybutylene terephthalate polymers which were first disclosed, along with polyethylene terephthalate in U.S. Pat. No. 2,465,319 to Whinfield and Dickson have been long associated with polyethylene terephthalate and believed, by those skilled in the art, to be equivalent with all polyalkylene terephthalates.
The other area of superior results attained by reinforced polypropylene terephthalate and polybutylene terephthalate compared to molding resins of the prior art, and especially reinforced polyethylene terephthalate, is the noticeably improved performance properties of the molded articles made of these resins. For example, reinforced polybutylene terephthalate resins although possessing equivalent strength compared to an equivalent reinforced polyethylene terephthalate resin has substantially higher toughness, lower water absorption, better creep properties and more desirable color properties than does an equivalently filled polyethylene terephthalate resin.
The above improved results attained with reinforced polypropylene terephthalate and polybutylene terephthalate compared to equivalent molding resins of the prior art as exemplified by polyethylene terephthalate explain the recent explosive growth in the use of these thermoplastic polyesters. A fundamental property necessary to further expansion in the utilization of these resins is tensile strength retention after long term exposure to high temperatures. This property is fundamental to the utilization of polypropylene terephthalate and polybutylene terephthalate in many applications requiring high strength properties.
Certain molded articles not only require high tensile strength but furthermore require that the molded article retain its strength even under long exposure to high temperatures. As those skilled in the art are aware, thermoplastic molding resins in general lose a considerable percentage of their tensile strength when exposed to high temperature over extended periods of time. This is true of polypropylene terephthalate and polybutylene terephthalate. Thus, although the addition of reinforcing fillers raise the tensile strength of these resins to a point where they became utilizable as engineering resins, still the problem of high temperature stability limits their utilization and bars their use in many important applications. For instance, there are many articles disposed under the hood of automobiles, which when fabricated of polypropylene terephthalate or polybutylene terephthalate represent an advance over the materials of construction currently employed. An example of such an article is a distributor cap. A distributor cap molded of glass reinforced polybutylene terephthalate has superior electrical properties compared to the thermoset distributor caps of the prior art. However, a problem associated with a glass reinforced polybutylene terephthalate distributor cap lies in the loss of tensile strength with time due to the high temperatures existing under the automobile hood.
The problem of heat stabilization of polypropylene terephthalate and polybutylene terephthalate has already been considered in the prior art. However, one is always looking for improvements in heat stabilization for longer periods of time so as to supply molding resins maintaining physical and chemical properties after long term exposure to elevated temperatures.